The patterning of radiation sensitive polymeric films with high energy radiation such as photons, electrons or ion beams is the principle means of defining high resolution circuitry found in semiconductor devices. The radiation sensitive films, often referred to as photoresists regardless of the radiation source, generally consist of multicomponent formulations that are usually spin-cast onto a desired substrate such as a silicon wafer. The radiation is most commonly ultraviolet light of the wavelengths of 436, 365, 257, 248, 193 or 157 nanometers (nm), or a beam of electrons or ions, or ‘soft’ x-ray radiation, also referred to as extreme ultraviolet (EUV) or x-rays. The radiation is exposed patternwise and induces a chemical transformation to occur that renders the solubility of the exposed regions of the films different from that of the unexposed areas when the films are treated with an appropriate developer, usually a dilute, basic aqueous solution, such as aqueous tetramethylammonium hydroxide (TMAH).
Photoresists are generally comprised of a polymeric matrix, a radiation sensitive component, a casting solvent, and other performance enhancing additives. The highest performing photoresists in terms of sensitivity to radiation and resolution capability are the group of photoresists termed “chemically amplified.” Chemically amplified photoresists allow for high resolution, high contrast and high sensitivity that are not afforded in other photoresists. These photoresists are based on a catalytic mechanism that allows a relatively large number of chemical events such as, for example, deprotection reactions in the case of positive photoresists or crosslinking reactions in the case of negative tone photoresists, to be brought about by the application of a relatively low dose of radiation that induces formation of the catalyst, often a strong acid. The nature of the functional groups that comprise the polymeric matrix of these photoresists dictates the tone of the photoresist (positive or negative) as well as the ultimate performance attributes.
The nature of the polymeric matrix also dictates the suitability of a given photoresist for exposure with particular radiation sources. That is, the absorbance characteristics of a polymer must be carefully considered when designing a material for lithographic applications. This is important with optical lithography where polymers are chosen to provide a relatively transparent matrix for radiation-sensitive compounds such as photoacid generators (PAGs). Absorbance characteristics are also important because the wavelength of radiation used in optical lithography is directly proportional to the ultimate resolution attainable with a photoresist. The desire for higher resolution causes a continuing drive to shorter and shorter radiation wavelengths. For example, the phenolic polymers used for 248 nm imaging, namely derivatives of poly(4-hydroxystyrene) or PHS, are unsuitable for use with 193 nm radiation as the opacity of these PHS materials at 193 nm does not allow for sufficient radiation to create an appropriate image profile throughout the photoresist film thickness. Therefore, material selection and creation is necessary for each wavelength of optical radiation used.
In addition to absorbance characteristics, another parameter to be considered in the design of new photoresist materials is the dissolution behavior of the material in the given developer. The semiconductor industry has largely supported the use of 0.263 normal (N) TMAH as a developer for photoresist. The aforementioned PHS materials used in 248 nm imaging have a distinct and beneficia31 property in that these materials tend to dissolve very uniformly in 0.263 N TMAH, without swelling. Additionally, the rate at which the polymeric films dissolve can be tuned by the use of, for example, protecting groups and dissolution inhibitors in positive tone photoresists, and by effective crosslinking and other functionalization in negative tone photoresists. This property of uniform dissolution has been a difficult property to incorporate into new photoresist materials, especially those designed specifically for 193 nm imaging. The current polymer platforms chosen for 193 nm imaging, such as acrylic acid derivatives, cyclic olefins and alternating cyclic olefin-maleic anhydride-based materials, generally fall into this category of nonlinear dissolution. In fact, these materials often exhibit significant swelling during the initial stages of development. This has made the development of photoresists based on these materials quite challenging, particularly so for negative tone formulations.
Alternative materials based on fluoroalcohols have been previously proposed as a means of providing aqueous base solubility. See, e.g., H. Ito et al., “Polymer Design for 157 nm Chemcially Amplified Resists,” Proc. SPIE, 4345:273–284 (2001); R. R. Kunz et al., “Experimental VUV Absorbance Study of Fluorine-Functionalized Polystyrenes,” Proc. SPIE, 4345:285–295 (2001); and Y. C. Bae et. al., “Rejunvination of 248 nm Resist Backbones for 157 nm Lithography,” J. Photopolym. Sci. Tech., 14:613–620 (2001). Examples of such materials include norbornene hexafluoroalcohol, styrene hexafluoroalcohol and cyclohexyldodecylfluoroalcohol-based polymers. While each of these platforms provides base-soluble materials, each has disadvantages for commercial high resolution photoresist applications. The norbornene hexafluoroalcohol monomer requires special polymerization conditions, such as ring-opening polymerization, transition metal catalyzed addition polymerization, or alternating free-radical polymerization with a comonomer such as maleic anhydride. Thus, this monomer does not accommodate a large number of suitable comonomers; a desirable property which allows for a large degree of variation in composition, and thereby, materials properties. The styrene hexafluoroalcohol-based polymers are not suitable for imaging with 193 nm radiation due to their opacity at this wavelength, as with other styrenic materials such as PHS. The cyclohexyldodecylfluoroalcohol acrylates suffer from their high degree of synthetic complexity thus rendering their manufacture prohibitively expensive.